Device and method for melting or refining glass or glass ceramics

ABSTRACT

The invention relates to a device for melting or refining glass or glass ceramics. According to the invention, a device of this type is provided with the following characteristics: a plurality of tubes which are U-shaped and arrange side by side so that they form a cage like skull channel that is open on top, and a high frequency oscillation circuit which comprises an induction coil. The tubes can be connected to a cooling medium. The induction coil wraps around the channel in such a manner that winding sections extend along the lateral walls of the channel.

FIELD OF THE INVENTION

The invention concerns a device for the melting or refining of glassesor glass ceramics.

BACKGROUND OF THE INVENTION

Such devices have become known in the configuration of the so-called“skull pot”. They comprise a pot walling. This is generally cylindrical.It is constructed of a crown of vertical metal pipes. Slots remainbetween adjacent pipes. The bottom of the pot can also be constructed ofmetal pipes. However, it can also consist of refractory material. Theends are connected to vertical pipes for the introduction of coolingagent or for the discharge of cooling agent.

Heating is conducted by means of an induction coil, which surrounds thepot walling and by means of which high-frequency energy can be inputinto the contents of the pot.

Such a skull pot has been made known, for example, from EP 0 528,025 B1.

A skull pot operates as follows: The pot is filled with a [fresh] glassbatch or refuse glass or a mixture thereof. The glass or the melt mustfirst be preheated in order to obtain a certain minimum conductivity.Preheating is primarily conducted by means of burner heating. If thetemperature for [HF energy] input has been reached, then further energyinput can be supplied by means of irradiation by high-frequency energy.During the operation, in addition to the high-frequency energy heating,the melt is also heated by means of burners, which operate from the toponto the melt, or by means of hot off-gases. This additional heating isparticularly necessary in the case of the use of a skull pot forrefining. That is, if the surface layer is cold and correspondinglyhighly viscous, then bubbles will be prevented from exiting the melt ora foaming will occur.

Usually, the skull pot is arranged in a standing position. It isgenerally operated discontinuously.

JP 57 [1982]-95,834 describes a device with a quartz channel, which isarranged horizontally.

A high-frequency oscillating circuit, which contains a cylindrical coil,is assigned a to the quartz channel. The cylindrical coil wraps aroundthe quartz channel. The quartz channel is actually cooled, However, whenaggressive glasses are melted, it does not have a high long-termstability and a high breaking strength. In addition, a special heatingof the melt surface is not possible. In fact, a certain cooling occurs,which can lead to the formation of a tough skin in the surface region.If such a channel is to be used as a refining device, then bubbles canno longer rise up unhindered and be discharged from the melt. Thechannel therefore cannot be used for refining. If the channel is usedfor melting, and the melt contains readily volatile components, thenthere is a risk of condensation at the cooled superstructure of thechannel. The condensate can thus drip into the melt in an uncontrolledmanner. This can lead to glass defects in the form of nodes, blisters orstreaks. If corrosion of the coil material occurs, then this leads todiscoloration of the glass, depending on the material of the coil. Thisis not acceptable, particularly in the case of optical glasses.

Further, there are very many optical glasses, which have a highproportion of fluorine, phosphate or other highly aggressive components.These can also attack the material of the coil. The corrosion can bestrong enough that discharge of cooling water occurs, so that theoperational safety of the plant is no longer assured.

SUMMARY OF THE INVENTION

The object of the invention is to create a device, in which theadvantages of the technique of inductive heating are utilized, which isreliable in operation, which is also suitable for the refining of melts,and which leads to glasses of a perfect quality. This object is resolvedby the features of claim 1.

According to the invention, not only is use made of the high-frequencytechnique, but also the skull technique is used. A channel is used,which has a structure similar to that of a skull pot. The upper space isnot covered by water-cooled pipes in this way. Rather, it is freelyaccessible and can be used for thermal insulation or for an additionalheating by means of a burner or by means of radiant heat.

The invention, however, introduces the following additional advantage,which the inventors have recognized:

If the water-cooled metal pipes of a skull device run in the directionof the glass flux, then flashovers between the glass melt and the metalpipes of the skull channel can occur at high melt temperatures, if thesolidified cold glass insulation layer is very thin. This can lead toarcing between the skull channel and the melt, which can have as aconsequence a disruption of the skull frame. It is presumed that the arcformation is produced by high-frequency voltages induced in the skullpipe.

In one embodiment according to the invention, the water-cooled metalskull pipes run perpendicular to the direction of glass flow, thus notin the direction of glass flow. In this way, the formation of arcsbetween the skull pipes and the melt is extensively avoided.

In another embodiment of the invention, the tendency toward flashover,i.e.: the tendency to form arcs, is fully prevented in that the ends ofthe U-shaped piece of the skull pipe are joined with each other in aconductive manner for purposes of forming a short-circuit bridge[shunt].

The invention introduces the following additional advantages:

It is excellently suitable for continuous operation. It can thus operatevery economically.

Another advantage consists of the following:

Due to the configuration and arrangement of the induction coils in thelying-down position, the channel is open at the top. The level of themelt is exposed. The surface of the melt is thus freely accessible forthe installation of an additional heating device, for example, a gasburner or an electrical heating device. This top heating is ofparticular advantage for the case when the channel is utilized as arefining aggregate. High surface temperatures can be obtainedaccordingly, so that the bursting of bubbles is assured in the region ofthe surface.

The heating from above is also helpful if high-frequency energy failureoccurs. In this way, at least the glass transport can be assured. Also,the melt temperature can be maintained at such a value that a recouplingis possible when high-frequency heating is again started up.

Further, there is no danger of condensation of products of evaporationon the water-cooled coil pipes, since these are not found above thelevel of the melt.

Additionally, a complex superstructure is provided in the case of theskull channel according to the invention, which includes ceramic platesthat cover thechannel. The ceramic plates can be heated on the top sideby means of burners. The plates then radiate heat onto the glass surfaceby their lower side, so that the glass is indirectly heated. This hasthe advantage that strong and turbulent atmospheric interferences do notoccur directly below^(*) the level of the glass melt in the case ofglasses containing components that have a high tendency towardevaporation (B₂O₃, P₂O₅, F, S, Se, Te or the like). Such interferencewould entrain the easily volatile components, which would lead to amodification of the glass composition. A premature blockage of filterdevices is also avoided in this way.

^(*) sic; above?—Trans. note.

Another advantage of the skull channel according to the invention liesin the fact that when additional heating is produced by means ofburners, with or without ceramic cover, a reducing atmosphere can beestablished. This is necessary for the production of thermal insulationglasses or glasses with high UV transmissivity, in which it happens thatthe Fe³⁺/Fe²⁺ ratio is shifted as extensively as possible to the reducedform. Fe²⁺, which absorbs in the IR, and thus is used for heat radiation(thermal insulation glass), whereas Fe³⁺, which absorbs in the UV, mustbe avoided as extensively as possible in the case of glasses with highUV transmissivity. Since these glasses are often phosphate orfluorophosphate glasses, the use of a ceramic cover plate is important.A similar argument applies to the production of initial glasses, inwhich it happens that the chalcogenides necessary for coloring arepresent at least partially in reduced form (S²⁻, Se²⁻, Te²⁻). Here, itis also of advantage to minimize evaporation, in this case of colorcomponents, by the use of ceramic cover plates.

Reducing conditions may also be established with the use of electricalheating from above by means of corresponding reducing gases or gasmixtures (forming gas, H₂, CO/CO₂ and others), but the use of anadjusted burner to produce a reducing atmosphere (incomplete gascombustion, i.e., a smaller quantity of air/oxygen) is generally morecost-favorable.

The described channel systems may be joined by flanges to conventionallyheated platinum or stone channels. When connected to a stone channel,the cooling of the stone channel-skull transition region is important.In operation, usually a good contacting of the water-cooled channel withthe stone material is sufficient. During the heat-up phase, the freedomof motion of the stone channel must be assured relative to the HFchannel, since the stone channel extends during tempering, whereas thewater-cooled HF channel retains its geometry. The procedure of movingthe stone channel up to the HF channel only after tempering andattaching it in the hot state has proven optimal.

When an HF channel is contacted with an electrically heated platinumchannel, it must be assured that either there is no electrical contactbetween the metal components of the HF channel or, however, there is avery good electrical contact. The latter case conceals the danger thatHF interference signals might be decoupled by means of the platinumsystem, but is preferred to the poor contact, which is accompanied byspark formation at places with increased resistance.

A complete electrical separation between skull channel and platinumchannel can be achieved by ceramic intermediate pieces, which mustassure a distance of at least 5 mm between metal components. Greaterdistances offer more security relative to electric breakdown strength,but are more difficult to seal, particularly in the case of aggressivemelts. A quartz ceramic has proven most suitable as insulation material.

If the channel has a length of more than 1200 mm, then it must be heatedwith several flat coils, whereby the flat coils are ideally providedwith energy by different HF generators, in order to be able to adjustthe temperature in the individual channel regions, independently of oneanother. The distance x between adjacent flat coils should be greaterthan or at least equal to the height of the coil winding d, and thus theHF fields cannot mutually influence one another.

An unheated or only very weakly heated region lies in the transitionregion between two flat coils, since the two flat coils cannot berandomly guided next to one another. The melt cools down in this zone.An up-and-down heating of a glass melt is undesired for glass quality,and particularly also due to the danger of thermal reboil. In order toassure a flat temperature profile or a monotonically rising ormonotonically falling temperature profile over the entire channellength, an additional heating device must be installed in the transitionregion between two coils. In the channel type described here, either anadditional electrical heating device (e.g., sic rods or kanthal needles)or a gas firing can be utilized. In the case of gas firing, the use offlat coils and guiding the coil just below the channel has provenadvantageous.

DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail on the basis of thedrawing. Here, the following is represented individually:

FIG. 1 is a top view onto an induction coil for a device according tothe invention.

FIG. 2 is a 3-D view of an induction coil, which is slightly curved inone plane.

FIG. 3 is a 3-D view of two coils, which are each slightly curved in oneplane.

FIG. 4 shows a cage-type skull channel.

FIG. 5 schematically illustrates a skull channel with several flat coilsconnected in series.

FIG. 6 shows a skull channel in a section perpendicular to the directionof glass flow with induction coil and burner belonging to it.

FIG. 7 shows a similar subject to that of FIG. 6, but with an additionalelectrical heating device.

DETAILED DESCRIPTION OF THE INVENTION

Coil 1 shown in FIG. 1 has endless-screw-shaped [helical] runningwindings 1.1, 1.2, 1.3. In the present case, the windings lie in ahorizontal plane, precisely in the direction of glass flow 2 (see FIG.2). The inside diameter of the inner winding in the direction of glassflow 2 is relatively large. It can amount to a multiple of the insidediameter perpendicular to the direction of glass flow 2.

The coil 1 shown in FIG. 2 is also shaped like an endless screw and haswindings 1.1, 1.2, 1.3. It is understood that a much larger number ofwindings is also possible. This coil is slightly curved in a plane. Thewinding segments running in the direction of glass flow 2 lie on bothsides of the channel, which is not shown here.

The windings are subdivided in the coil shown in FIG. 3. Windingsegments are again recognized, which run in a straight line parallel tothe direction of glass flow. The curved winding segments lie at thebeginning and the end of the channel. One-half of the windings run belowand one-half of the windings run above the channel, which is not shown.In this way, the following is achieved: those high-frequency stresses,which are induced in skull pipes and which are produced by curved coilsegments, are extensively neutralized by the countercurrent circuit ofthe curved winding segments.

FIG. 4 shows the skull channel 3. It has a multiple number of U-shapedskull pipes 3.1-3.7. The skull pipes lie in planes parallel to oneanother. Instead of a pure U-shape, deviations from this are alsoconceivable, for example, an approximate V shape. The skull pipes are,as in the case of skull pots, water-cooled metal pipes.

Conductors 4 are provided at the free ends of the U-shaped elements, andthese shunt the free ends of the U elements. These shunt lines 4 arealso cooled by air or water.

In the present case, the U-shaped elements run in planes, which lieperpendicular to the direction of glass flow 2. However, it would alsobe conceivable to arrange the U-shaped elements in planes inclined tothis direction.

FIG. 4 makes it clear that the space enclosed by shunt conductors 4 isopen toward the top. The melt is thus accessible from the top, exceptfor the shunt zones at the channel inlet and at the channel outlet.Thus, there are no water-cooled components above the melt and there isalso no danger of condensation of evaporation products with thedisadvantages described above. Also, gas burners or other additionalheating devices can be arranged above the melt. Heat from above isadvantageous for the case when the channel is utilized as a refiningaggregate. This additional heating may be necessary in order to bringthe surface region of the melt to particularly high temperatures, andthus the bursting of bubbles and the discharge of gas from the melt isassured.

FIG. 5 shows a relatively long skull channel 3. Several flat coils 1,10, 100, are assigned to this channel 3. Also, additional heatingdevices 5.1, 5.2 are provided. The additional heating devices each timelie in the transition region between two flat coils.

FIG. 6 shows a device according to the invention in a sectionperpendicular to the direction of glass flow. As is shown in FIG. 4,melt 8 flows through skull channel 3. Thus the melt flow movesextraordinarily slowly. The skull channel is surrounded by an inductioncoil 1. This may have the configuration of the coils shown in FIGS. 1-3.

The upper furnace space is formed of a structure 6 of refractorymaterial. An additional burner heating unit 5.3 is provided therein. Thelatter can transfer heat directly onto the melt surface. However, thetransfer may also be made indirectly. As shown here, a ceramic plate 7can be provided, which is heated by the burner additional heating unitand then heat is introduced, distributed uniformly on the melt surface.

In the form of the embodiment according to FIG. 7, instead of a ceramicplate 7, an additional electrical heating unit 5.4 is provided, whichheats the melt surface.

The coil has a central opening that is as large as possible The coilruns to the right and left of the channel parallel to the glass flow andat the end of the channel, below the channel, onto the opposite-lyingside of the channel. Ideally, one-half of the windings run below thechannel and the other half of the windings run above the channel on theopposite-lying side. It is achieved in this way that the HF voltagesinduced by these coil pieces in the skull U-shaped pipes are extensivelyneutralized by the countercurrent circuit. In the region of the coilfeedback on the opposite-lying channel side, the skull channel isshunted at the upper end from one side of the channel to the other. Theshunt is cooled by air or water.

The skull channel preferably comprises a number of U-shaped segments,which have a circuit shunt at the upper end. In projection from the top,the coil is a helical, wound, rectangularly crushed flat coil, whosenarrow sides are guided around above and/or below the channel. If thecoil pieces are guided along above the channel, then ceramic insulation,e.g., in the form of a quartz bridge can be introduced between the meltand the coil.

The construction has the advantage, when compared with cylinder-shapedchannels with cylindrical coils, that no water-cooled components arepresent in the upper region of the melt, with the exception of the shuntzones at the inlet and outlet of the channel, so that the melt is hotterhere and there is no danger of condensation of evaporation products.Also, the region above the melt is freely accessible for theinstallation of a gas or electric upper heating unit. This upper heatingunit is advantageous for the case when the channel is used as a refiningaggregate, since higher surface temperatures can be obtained therewith,and thus the bursting of bubbles can be assured. Upper heating is alsohelpful in the case of the failure of high-frequency energy, since inthis case at least the glass transport can be assured and recoupling ofhigh-frequency heating is facilitated after the failure.

In addition, the described structure is advantageous for introducing acomplex superstructure, comprised of ceramic plates, which cover thechannel, in which the gas flows. These ceramic plates are heated byburners on the upper side and in turn radiate the glass surface by theirunderside, so that the glass is indirectly heated. This has theadvantage that in glasses containing components tending strongly towardevaporation, such as, for example, B₂O₃, P₂O₅, F, S, Se, Te and others,there is no occurrence of strong and turbulent atmospheric flowsdirectly above the glass melt, which entrain the easily volatilecomponents and thus lead to a modification of the glass composition.Also, a premature blockage of filter devices caused by this is avoided.

Another advantage of the selected structure is that a reducingatmosphere can be established with an additional heating by means ofburners, either with or without ceramic cover plates. This is necessaryfor the production of thermal insulation glasses or glasses with high UVtransmissivity, in which it happens that the Fe²⁺/Fe²⁺ ratio is shiftedas extensively as possible to the reduced form. Fe²⁺, which absorbs inthe IR, thus is used for heat radiation (thermal insulation glass),whereas Fe³⁺, which absorbs in the UV, thus must be avoided asextensively as possible in the case of glasses with high UVtransmissivity. Since the glasses are often phosphate or fluorophosphateglasses, the use of a ceramic cover plate can be important. A similarargument applies to the production of initial glasses, in which ithappens that the chalcogenides necessary for coloring are present atleast partially in reduced form (S²⁻, Se²⁻, Te²⁻). Here, it is also ofadvantage to minimize evaporation, in this case of color components, bythe use of ceramic cover plates.

The present invention having been thus described with particularreference to the preferred forms thereof, it will be obvious thatvarious changes and modifications may be made therein without departingfrom the spirit and scope of the present invention as defined in theappended claims.

What is claimed is:
 1. A device for melting or refining of glasses orglass ceramics comprising: a plurality of pipes forming a U-shape andlying next to one another so that said plurality of pipes form acage-type skull channel having an open top, said plurality of pipesbeing able to be connected to a cooling medium, said cage-type skullchannel for channeling a melt of the glasses or glass ceramics in asubstantially horizontal flow direction; and a high-frequencyoscillation circuit having an induction coil, said induction coil beingdisposed about a portion of said cage-type skull channel such that saidopen top is free of said induction coil.
 2. The device according toclaim 1, wherein said U shape has ends, and wherein the ends of theU-shape are joined together in a conducting manner for purposes offorming a short-circuit bridge.
 3. The device according to claim 1,wherein said cage-type skull channel is thermally insulated in an upperspace of a furnace.
 4. The device according to claim 1, furthercomprising an additional heating unit in an upper furnace space.
 5. Thedevice according to claim 4, wherein the additional heating unit isconfigured and arranged to act directly on a surface of said melt. 6.The device according to claim 4, further comprising a ceramic plate thatis heated by said additional heating unit and gives off heat to asurface of said melt between said additional heating device and thesurface of said melt.
 7. The device according to claim 1, furthercomprising a plurality of flat coils connected one behind the other andassigned to said cage-type skull channel.
 8. The device according toclaim 7, further comprising an additional heating unit provided in atransition region defined between said plurality of flat coils and asecond plurality of flat coils.